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Customer-specific plans will require you to manage a bunch of plans for each customer you negotiate and override the price with - it is feasible but you should plan to manage a bunch of plans per customer.
Customer-specific coupons will show up in the invoice along with the original price, so it is not a good option to show a discount when a discount is not actually applied. Additionally, managing multiple coupons per customer will also be a challenge as different coupon value discounts should be created and managed for customers.
Adding $1 Addons of 'x' quantity is not a viable option either. Imagine a $399 plan is overridden to $349 for a customer, then 349 * $1 addons should be added to the subscription.
The Price Override feature allows you to enter a custom price - a clean approach to managing and displaying custom prices per customer in the invoices.
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